Systematic paleontology of macroalgal fossils from the Tonian Mackenzie Mountains Supergroup

Abstract

Proterozoic eukaryotic macroalgae are difficult to interpret because morphological details required for proper phylogenetic studies are rarely preserved. This is especially true of morphologically simple organisms consisting of tubes, ribbons, or spheres that are commonly found in a wide array of bacteria, plants, and even animals. Previous reports of exceptionally preserved Tonian (ca. 950−900 Ma) fossils from the Dolores Creek Formation of Northwestern Canada feature enough morphological evidence to support a green macroalgal affinity. However, the affinities of two additional forms identified on the basis of the size distribution of available specimens remain undetermined, while the presence of three unique algal forms supports other reports of increasing algal diversity in the early Neoproterozoic. Archaeochaeta guncho new genus new species is described as a green macroalga on the basis of its well-preserved morphology consisting of an unbranching, uniseriate thallus with uniform width throughout and possessing an elliptical to globose anchoring holdfast. A larger size class of ribbon-like forms is interpreted as Vendotaenia sp. A third size class is significantly smaller than Archaeochaeta n. gen. and Vendotaenia, but in the absence of clear morphological characters, it remains difficult to assign. As Archaeochaeta n. gen. and Vendotaenia represent photoautotrophic taxa, these findings support the hypothesis of increasing morphological complexity and phyletic diversification of macroalgae during the Tonian, leading to dramatic changes within benthic marine ecosystems before the evolution of animals.

Phylotranscriptomics unveil a Paleoproterozoic-Mesoproterozoic origin and deep relationships of the Viridiplantae

“…The origin of prasinophytes was also dated back to between the middle Paleoproterozoic and middle Mesoproterozoic (1.87–1.22 Ga), followed by a Mesoproterozoic origin of core Chlorophyta (1.65–1.08 Ga) (Supplementary Table 2 ). In addition to macrofossils like P. antiquus and P. sinense with specific taxonomic explanations, there are well-preserved fossils from the Dolores Creek Formation (0.95–0.90 Ga) that exhibit characteristics of green macroalgae 70 – 72 . Moreover, the macrofossil Horodyskia from the Tonian successions (0.95–0.72 Ga) in North China was tentatively interpreted as a multinucleate (coenocytic) green macroalga 73 .…”

1.63-billion-year-old multicellular eukaryotes from the Chuanlinggou Formation in North China

“…Fossils found on several continents show that in the oceans, simple multicellular eukaryotes, such as uniseriate filaments and coenobia, arose long before the advent of complex multicellular animals and algae ( 2 , 3 ); prokaryotic multicellularity extends even further back into the Archean ( 4 ). Relatively abundant late Mesoproterozoic to early Neoproterozoic populations include forms interpreted as red [ Bangiomorpha pubescens , ~1050 million years (Ma), arctic Canada ( 5 , 6 )] or green [ Proterocladus antiquus , ~950 Ma, North China ( 7 )] algae, as well as putative early fungi [ Ourasphaira giraldae , ~890 Ma, Arctic Canada ( 8 )] and eukaryotic problematica, including Eosolena loculosa [~1030 Ma, Siberia ( 9 )], Arctacellularia tetragonala [~1000 Ma, Congo ( 10 )], and Archaeochaeta guncho [~950 Ma, northwestern Canada ( 11 )]. Less common records of both cellularly preserved microfossils such as Eosolena minuta from northern Siberia ( 12 ) and decimeter-scale carbonaceous compressions from North China ( 13 ) extend the record of eukaryotic multicellularity back to the early Mesoproterozoic era.…”

The paleoredox context of early eukaryotic evolution: insights from the Tonian Mackenzie Mountains Supergroup, Canada

“…Here, we present results of a multi-proxy geochemical investigation that includes iron speciation data, redox-sensitive trace element abundances and Nd-Sm data from ca. 1000-850 Ma shales in the lower MMS in the Wernecke Mountains, including rocks from which macroalgal fossils have been reported (Maloney et al, 2021(Maloney et al, , 2023.…”

“…1000–800 Ma redox conditions coincident with multicellular eukaryotic evolution. In particular, MMS features a diverse macroalgal assemblage including large (cm‐scale) green macroalgae Archaeochaeta , found in the Hematite Creek Group (Maloney et al., 2023), the carbonaceous macrofossils Chuaria and Tawuia (Hofmann, 1985; Hofmann & Aitken, 1979) and purported poriferan body fossils (Turner, 2021) within reefal facies in the Little Dal Group. Previous redox studies on the Cryogenian to Ediacaran Windermere Supergroup in the Wernecke Mountains and equivalent strata in the Mackenzie Mountains have provided evidence for a generally anoxic, ferruginous basin (Johnston et al., 2013; Miller et al., 2017; Shen et al., 2008; Sperling et al., 2016).…”

“…Here, we present results of a multi‐proxy geochemical investigation that includes iron speciation data, redox‐sensitive trace element abundances and Nd‐Sm data from ca. 1000–850 Ma shales in the lower MMS in the Wernecke Mountains, including rocks from which macroalgal fossils have been reported (Maloney et al., 2021, 2023). Geochemical characterisation of these fossiliferous sections aids in reconstructing the paleoenvironmental conditions in which these primary producers diversified and provides insight into the relationship between eukaryotic expansion and environmental conditions during this critical transition in Earth's history.…”